The present invention is directed to improvements in molded golf ball construction and more particularly to improvements in molded golf ball core construction. The improved core is useful in producing balls having, among other things, superior sound and feel as well as enhanced playability characteristics. The present invention is also directed to the novel methods used in constructing the core and to golf balls produced utilizing the improved core construction.
Sound and feel are two qualities of golf balls which are typically judged subjectively. For the most part, however, soft sound ("click") and soft feel (i.e., low vibrations) are golf ball qualities desired by many golfers. If a soft feeling ball is mis-hit, the sting in the hands is not as great as if a harder feeling ball is hit improperly. A soft sounding ball has a soft low pitch when hit with any club, but particularly off a putter.
One way to achieve a soft sound and feel is to provide a softened layer between the core and the cover. The prior art teaches development of a three piece ball or a multi-layer cover. However, adding additional layers is costly and can sometimes lead to non-uniform layers.
The Molitor, et al. U.S. Pat. No. 4,650,193 describes a two-piece golf ball comprising a core and a cover. The core has a central portion of a cross-linked, hard, resilient material and a soft, deformable outer layer. The cover is a conventional cover. The soft, deformable outer layer of the core is integral with the core. It is formed by treating a slug of an elastomeric material with a cure altering agent, namely elemental powdered sulfur, so that a thin layer of sulfur coats the surface. The sulfur-coated slug is then cured in a molding cavity at temperatures greater than 290.degree. F., e.g., 325.degree. F., for 10-20 minutes, depending on core temperature.
According to the '193 patent, sulfur on the surface of the slug penetrates a surface layer to a depth of about 1/16 inch during curing. Wherever the core is exposed to sulfur, the conventional peroxide cure is altered, resulting in an amorphous soft outer layer. The portion of the core that is not touched by the sulfur cures normally and becomes relatively crystalline. The end result is a spherical core having a hardness gradient in its surface layers.
The present inventors seek to achieve somewhat of a similar effect using methods which do not require the addition of elemental sulfur to modify and soften the core surface such that the cure on the core surface is retarded. At the same time, the inventors seek to maintain the parameters of resilience and hardness of the finished ball at desired levels.
Resilience is determined by the coefficient of restitution (C.O.R.), the constant "e", which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. "e") can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. Riehle compression) of the ball under a fixed load of 200 pounds applied across the ball's diameter (i.e. the lower the compression value, the harder the material).
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball's incoming and outgoing velocity electronically. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (U.S.G.A.).
Along this line, the U.S.G.A. standards indicate that a "regulation" ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball's initial velocity (i.e. as the C.O.R. of a ball is increased, the ball's initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
An object of this invention is to develop a method for improving the sound and feel of a golf ball without adversely affecting the resilience or coefficient of restitution of the ball. The method does not require the addition of sulfur based chemicals to an uncured slug, in order to minimize the steps involved. In addition, the softer golf ball produces the playability characteristics desired by the more skilled golfer. It also enhances durability characteristics, as the outer skin is flexible and resists crack propagation.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.